Remove the need for HostRef<Module>

This commit continues previous work and also #708 by removing the need
to use `HostRef<Module>` in the API of the `wasmtime` crate. The API
changes performed here are:

* The `Module` type is now itself internally reference counted.
* The `Module::store` function now returns the `Store` that was used to
  create a `Module`
* Documentation for `Module` and its methods have been expanded.

crates/misc/py/src/module.rs

@@ -4,5 +4,5 @@ use pyo3::prelude::*;
 
 #[pyclass]
 pub struct Module {
-    pub module: wasmtime::HostRef<wasmtime::Module>,
+    pub module: wasmtime::Module,
 }